Tuesday 29 June 2010 19.05 EDT
First published on Tuesday 29 June 2010 19.05 EDT

People who live in countries where disease is rife may have lower IQs because they have to divert energy away from brain development to fight infections, scientists in the US claim.

The controversial idea might help explain why national IQ scores differ around the world, and are lower in some warmer countries where debilitating parasites such as malaria are widespread, they say.

Researchers behind the theory claim the impact of disease on IQ scores has been under-appreciated, and believe it ranks alongside education and wealth as a major factor that influences cognitive ability.

Attempts to measure intelligence around the world are fraught with difficulty and many researchers doubt that IQ tests are a suitable tool for the job. The average intelligence of a nation is likely to be governed by a complex web of interwoven factors.

The latest theory, put forward by Randy Thornhill and others at the University of New Mexico, adds disease to a long list of environmental and other issues that may all play a role in determining intelligence. Thornhill made the news in 2000, when he coauthored a provocative book called A Natural History of Rape in which he argues that sexual coercion emerged as an evolutionary adaptation.

To test the idea, Thornhill's group used three published surveys of global IQ scores and compared them with data from the World Health Organisation (WHO) on how badly infectious diseases affect different countries. The list included common infections, such as malaria, tetanus and tuberculosis.

The scientists found that the level of infectious disease in a country was closely linked to the average national IQ. The heavier the burden of disease, the lower the nation's IQ scores. Thornhill believes that nations who have lived with diseases for long periods may have adapted, by developing better immune systems at the expense of brain function.

"The effect of infectious disease on IQ is bigger than any other single factor we looked at," said Chris Eppig, lead author on the paper. "Disease is a major sap on the body's energy, and the brain takes a lot of energy to build. If you don't have enough, you can't do it properly."

"The consequence of this, if we're right, is that the IQ of a nation will be largely unaffected until you can lift the burden of disease," Eppig added.

"The authors suggest that more infectious disease could lead to lower IQ scores through an impact on brain development. This is an interesting speculation, but the data don't prove it one way or the other," he said. "A bigger problem is that it might be driven by a third factor, that affects both infectious disease prevalence and IQ test scores."

For reasons that are unclear, IQ scores are generally rising around the world. Thornhill suggests monitoring rates of infectious diseases in nations as they develop, to see if they decline and IQ tests scores rise.

"In recent decades, HIV has been a serious infectious disease, and it has a high infection rate in low IQ countries, especially in southern Africa, where it is present in around 30% of the population … This is attributable to the low IQ of the population who do not understand the way the infection is contracted, and have erroneous beliefs about how to prevent infection."